The use of high frequency attenuation cables has sharply increased in recent times. High frequency attenuation cables which also protect against electromagnetic interference (hereinafter EMI) are especially desirable for military applications. Light weight, labor efficient high frequency attenuation cables are especially valuable for use on board fixed wing aircraft and helicopters and the like.
There have been many high frequency attenuation cables in the past. The more relevant of these structures are discussed in commonly assigned U.S. patent application Ser. No. 210,202, now U.S. Pat. No. 4,347,487 (hereinafter Martin) which is incorporated herein by reference. Martin discloses a high frequency attenuation cable having an EMI shield, which when bundled with other similar cables eliminates most sneak path problems. It is well established that sneak paths dilute the effectiveness of high frequency attenuation. In typical multi-conductor or harness applications of high frequency attenuation cables having an EMI shield, a sneak path is created between adjacent cables' EMI shields and their surrounding dielectric. The sneak path allows high frequency energy filtered by the respective cable's attenuation layer to jump from the attenuation layer and travel along the EMI shield.
Martin is directed toward eliminating the problem of sneak paths in multi-conductor or harness cables. Martin discloses a structure having a standard core for high frequency attenuation cables consisting of a conductor surrounded by a high frequency attenuation medium, and a dielectric layer which further surrounds the attenuation medium. Martin further includes the standard core surrounded by an EMI shield, which is further surrounded by a conductive outer layer. The conductive outer layer acts to cancel sneak paths of multi-conductor or harness cables by shorting out the sneak paths of known core consisting of a central conductor of each high frequency attenuation cable against the other cable's conductive outer layer. The individual cable in accordance with Martin does not in itself produce better high frequency attenuation efficiently than previously known high frequency attenuation cables. Rather, when Martin cables are combined in a multi-conductor or harness applications, the resultant structure retains almost all the high frequency attenuation efficiency of the individual cable which it would otherwise lose due to sneak paths.
The instant invention discloses a particularly high performance, high frequency attenuation core. The core of the cable is that portion of the cable surrounded by the EMI layer, as will be explained more fully hereinafter. The individual core of the instant invention includes an additional layer of material surrounding the dielectric of the known core. The additional layer is preferably conductive but must at least possess the property of having a high complex dielectric constant (e.g. .epsilon..gtoreq.11).
The instant invention, in one embodiment, utilizes the discoveries set forth in Martin for producing a particularly good EMI shielded multi-core cable. In that embodiment, the individual core members each are surrounded by an EMI shield. The instant invention also includes another embodiment, wherein a gross shield is wrapped around a plurality of the above described cores to produce a lightweight, labor efficient cable.